Not Applicable.
Not Applicable.
The present invention relates in general to luminance control of fluorescent lamps, and, more specifically, to compensating for offsets and temperature variations in pulse-width modulation (PWM) circuits controlling lamp dimming.
Backlight display devices are used in a variety of consumer and industrial products to display data, charts, graphs, messages, other images, information, and the like. Backlight display devices have a backlight positioned to floodlight a display panel from the front or back. The backlight may be a fluorescent tube, an electroluminescent device, a gaseous discharge lamp, a plasma panel, and the like. The display panel display may be a passive or active matrix liquid crystal display (LCD), for example. The backlight and display panel are connected to control circuitry for providing a variable supply voltage in order to control brightness of the illumination. The display device may be separate or incorporated with other components, such as an electronic device in a dashboard of an automobile or other vehicle, a portable electronic device, and the like.
To control brightness, a driver circuit increases or decreases the drive current supplied to the backlight. The drive current typically is adjusted in relation to the environment (e.g., ambient lighting conditions) and user preferences. A lowly-lit environment usually requires less brightness, and thus a lower drive current, than a brightly-lit environment. The brightness may be changed automatically in response to the environment and/or manually. The backlight display device may have a switch, a keypad, a touch screen, a remote device, or the like to adjust the brightness.
Cold cathode fluorescent lamps (CCFLs) have been used as a backlight for LCDs. CCFLs are well suited to this application due to their low cost and high efficacy. High efficacy, which is equal to the ratio of light output to input power, is required because typical LCDs only transmit about 5% of the backlighting due to absorption of light in the polarizer and color filter of the LCD. In order to produce usable daytime lighting levels of approximately 400 Nits, the backlight must be capable of 20xc3x97400 Nits. One Nit is the luminance of one candle power measured one meter away over a meter by meter area, also known as a candela per meter squared. A cost effective backlighting technology which can provide such a lighting level is a fluorescent lamp.
Although the CCFL is an extremely efficient light source, it is difficult to control its illumination down to the low dimming levels required by, for example, night-time automotive environments. In some automotive applications, dimming at a barely discernable level (e.g., in the range of 1.0 Nit for an active matrix LCD) may be required. Accordingly, the CCFL controller must be capable of producing a dimming ratio of 400:1.
Most CCFL controllers have difficulty in controlling the absolute luminance down to this level. Some known systems obtain the desired dimming ratio by overdriving the lamp. However, this rapidly reduces the operating life of the lamp. Some military LCD systems use a first lamp for daytime illumination and a second, smaller lamp to produce the required night time lighting levels. However, systems which utilize dual lighting sources are not cost competitive in the automotive environment. Not only is a second lamp required, but a second controller is required as well.
Many control schemes have been used to control fluorescent lighting. Examples include voltage controlled self-resonant oscillators, pulse-by-pulse current pulse width modulated (PWM) control and PWM duty cycle control systems or combinations thereof. Pulse-by-pulse current PWM control systems characteristically operate at a frequency of 20 KHz to 100 KHz to control the lamp current. PWM duty cycle control of the CCFL luminance is accomplished by duty cycle control of the lamp""s on time to the total periodic update time. For example, a PWM signal may be generated having a frequency of about 120 Hz and a duty cycle ranging from 100% down to less than 1%. During the xe2x80x9conxe2x80x9d time of the PWM signal, a higher frequency (e.g., about 60 KHz) current supply is applied to the CCFL. The average drive current, and thus the total illumination, are reduced as the duty cycle is reduced.
While the backlight luminance is generally proportional to the drive current, the efficiency of the backlight may change during operation of the backlight display device. The changing efficiency varies the backlight luminance and hence the brightness of the backlight display device. The efficiency of the backlight display device usually is low at start-up and then increases during a xe2x80x9cwarm-upxe2x80x9d period. Even after the warm-up period, the efficiency of the backlight may change during operation of the backlight display device, such as when the backlight display device moves through colder and warmer ambient conditions. The backlight efficiency may change due to the drive current level itself. Higher drive currents tend to increase the lamp temperature and lower drive currents tend to decrease the lamp temperature, thus changing the efficiency. The backlight efficiency also may change for other reasons such as little or no lumen maintenance over time and variations in thermal resistance and circuit operation.
U.S. Pat. No. 6,388,388, issued to Weindorf et al, discloses a brightness control system for a backlight display device that measures the efficiency of the backlight in order to achieve a desired brightness or luminance for the backlight display device. The backlight efficiency is a function of the lamp temperature. At each lamp temperature, the luminance is linearly proportional to a desired drive current for the backlight. By using the measured lamp temperature and known backlight efficiency to derive a desired lamp current and then controlling the PWM duty cycle to generate the desired lamp current, the brightness control system may maintain the desired brightness throughout the dynamic range of the backlight display device. U.S. Pat. No. 6,388,388 is incorporated herein by reference in its entirety.
CCFL drive current may be controlled using an integrated circuit inverter such as a direct drive, non-resonant, PWM controller. The LX1686 Direct Drive CCFL Inverter produced by the Linfinity Division of Microsemi Corporation is one example. The desired lamp current may be computed in a digital microcontroller in response to a digitized lamp temperature measurement. This lamp current value is converted to an analog signal having a magnitude that corresponds to a PWM duty cycle of the inverter that creates the desired average lamp current. The analog signal is coupled to the IC inverter as a brightness command.
Although the lamp current that is necessary in order to create the desired illumination is known, it has been found that errors in actual illumination level continue to occur. Furthermore, the errors are not consistent from device to device. It has been discovered that temperature variations, other offsets, and noise effects associated with the inverter IC and its external components cause variations in the transfer function associating the analog brightness command to the actual lamp current produced. For example, a ramp generator used to generate a PWM signal may exhibit drift over temperature or the input power supply may vary.
The present invention has the advantage of accurately maintaining a commanded lamp current without temperature measurement or compensation of the inverter components themselves. A closed loop feedback current control system corrects for the current errors no matter what their cause.
In one aspect of the invention, a lamp brightness control for a lamp provides backlight illumination for a display. A brightness-to-current translator generates an electrical current command having a magnitude proportional to a desired lamp current that corresponds to a desired brightness. A PWM generator generates a PWM drive signal having a duty cycle determined in response to a control signal. A lamp driver switches power to the lamp in response to the PWM drive signal. A current sensor generates a current feedback signal in response to a flow of current in the lamp. An error amplifier generates the control signal in response to the electrical current command and the current feedback signal, whereby an actual lamp current is substantially equal to the desired lamp current despite any offsets in the PWM generator or the lamp driver.